Engine harmonic cancellation systems are adaptive feed-forward noise reduction systems that are used in motor vehicles, for example in cabins or in muffler assemblies, to reduce or cancel engine harmonic noise. Engine harmonic cancellation systems often use one or more microphones as input transducers. A sine wave at the frequency to be cancelled is also used as an input to an adaptive filter. The adaptive filter can alter the magnitude and/or the phase of the input sine wave. The output of the adaptive filter is applied to one or more transducers that produce sound (i.e., loudspeakers) that is acoustically opposite to the undesirable engine harmonics that are to be canceled. The aim of the system is to cancel the microphone signal at the frequency or frequencies of interest. In order to do so, the loudspeaker outputs have a negative gain.
Because the transfer function of the volume in which noise is being cancelled can change over time, such engine harmonic cancellation systems can significantly amplify noise at frequencies that are close to the frequency of the input sine wave. Schemes that are aimed to mitigate this noise amplification problem result in reduction of the cancellation performance of the system.